Presently known processes for synthesizing phosphate esters of ethylene glycol take an unusually long period of time and consume substantial amounts of energy. They also require relatively expensive reactants. For example, P. Carre ("Sur les ethers phosphoriques du glycol", Compt. Rend. 138:374-75 (1904) reported an esterification process which reacts ethylene glycol and phosphoric acid under low pressure. The Carre procedure involved heating an equimolecular mixture of such reactants using a vertical condenser at 140.degree.-145.degree. C. under vacuum pressures of 15 to 18 mm of Hg. The Carre process required approximately 10 hours to complete the reaction. A mixture of 3.5% of the triester, 43% to 57% of the diester, and 44% of the monoester obtained.
Bailly and Gaume ("Sur l'action de quelques halohydrines sur le phosphate neutre de sodium en solution aqueuse et sur quelques glycophosphates", Compt. Rend. 178:1191-93 (1924)) reported the preparation of sodium ethylene glycol phosphate by reacting an equimolecular mixture of ethylene glycol monochlorohydrin with neutral sodium phosphate in an 0.5N aqueous solution at room temperature. This procedure required 264 hours of reaction time to attain 81.5% conversion of the ethylene glycol monochlorohydrin to Na.sub.2 PO.sub.4 C.sub.2 H.sub.4 OH.
Ethylene glycol phosphate in the barium form was prepared by Plimmer and Burch (Esters of phosphoric acid, Part I, Chem Soc., London 1929:279- 291). Their method involved first preparing chloroethyl phosphoric ester from phosphoryl chloride and ethylene chlorhydrin, and then converting the resulting choloroethyl phosphoric ester into a barium phosphate ester of ethylene glycol (OH--C.sub.2 H.sub.40 --PO.sub.3 Ba). By a similar described procedure, a mixture of monoesters and diesters was prepared. The described methods are cumbersome and lengthy.
Atherton et al. (J Chem. Soc. 1945:382) prepared sodium ethylene glycol phosphate by reacting ethylene oxide and disodium hydrogen phosphate. At first glance, the method appears to be simple; however, it has drawbacks. For example, ethylene oxide is a carcinogenic substance requiring precautions for its handling. Also, less than desirable conversion of the ethylene oxide would be expected. Further, the requirement for the prior synthesis of disodium hydrogen phosphate would limit the use of this method in industrial processes.
The simplest of the prior art procedure appears to be the esterification of orthophosphoric acid with ethylene glycol (so called "direct esterification"). The one taught by Carre and described above is one version of this procedure. However, the prolonged heating requirement is a major drawback and prevents any practical commercialization of the process. Attempts by the applicants to speed the reaction by raising the temperature have proven to be ineffective since higher temperature causes carbonization of the reaction mixture with very poor yield and degradation of the ethylene glycol phosphate esters being sought.
Another version of the "direct esterification" procedure for making phosphate esters for use as a fertilizer, but as a fertilizer applied to the soil, is disclosed in Soviet Union patent 566,809 to Borisov which discloses phosphate esters for application to the soil and a method of making them. The method by Borisov consists of heating an equimolecular mixture of phosphoric acid and ethylene glycol for 3 to 4 hours, cooling and neutralizing it with 50% KOH and then drying at 90.degree.-100 degrees C.
Neither the patent to Borisov nor the publication of P. Carre disclose use of a phosphate ester as a foliar fertilizer and the methods they disclose for the synthesis of their phosphate ester are time-consuming and have low yields. The yields from the Borisov procedure are 60% or less of esterification. Most of the esterification is produced in the first 3-4 hours of heating. If additional esterification is desired at 90 degrees C. during drying, many hours of heating at 90 degrees is necessary making the procedure uneconomical.
Foliar application of phosphorus fertilizers is known and has been used for many years in spite of certain difficulties under some circumstances, such as for example: (1) leaf damage (burning) is caused by the phosphorus fertilizer solutions (osthosphosphates leads to burning problems and thus needs to be avoided); (2) there is poor absorption of phosphorus bearing compounds through the waxy cuticle of the leaf; (3) some phosphorus fertilizers have low mobility of within the plant and are unable to translocate from the leaf to other parts of the plant through the narrow phloem vessels; (4) there is a tendency to cause nutritional imbalances within the plant; and (5) when used at low application rates such as 10 or less gallons per acre in solutions diluted to avoid leaf damage, such as for cereals, little nutritive value is associated with the fertilizer.